1. Field of the Invention
The present invention relates to a copolymer for positive type lithography used in semiconductor production, a radical polymerization initiator used in production of said copolymer, and a composition for semiconductor lithography. More particularly, the present invention relates to a copolymer for chemically amplified positive type lithography using a radiation (e.g. deep-ultraviolet, X-rays or electron beam) and suited for microfabrication, a polymerization initiator used in production of said copolymer, and a composition for semiconductor lithography.
2. Description of the Prior Art
In the lithography employed for semiconductor production, formation of a finer pattern is necessary with an increase in integration degree. In the formation of a fine pattern, a light source of short wavelength is essential. Currently, a lithography using a Krypton fluoride (KrF) excimer laser beam (wavelength: 248 nm) is the main lithography used in semiconductor mass production and, also, a lithography using an argon fluoride (ArF) excimer laser beam (wavelength: 193 nm) is being introduced in semiconductor mass production. Further, lithographies using a fluorine dimer (F2) excimer laser beam (wavelength: 157 nm), extreme ultraviolet radiation (EUV), X-rays, electron beam or the like are being developed.
In these lithographies, copolymers for chemically amplified positive type lithography are being used suitably. These polymers are constituted so as to contain the following recurring units:
a recurring unit having a structure wherein a polar group soluble in alkaline developing solution (the group may be hereinafter referred to as “alkali-soluble group”) is protected by a substituent group which is unstable to acid and suppresses the solubility in alkaline developing solution (the substituent group may be hereinafter referred to as “acid-dissociating, dissolution-suppressing group”) (the above structure may be hereinafter referred to as “acid-dissociating structure”), and
a recurring unit having a polar group for increasing the adhesivity to semiconductor substrate or the like or for controlling the solubility in lithography solvent or alkaline developing solution.
In, for example, the lithography using a KrF excimer laser as the illuminant, there are known copolymers having a recurring unit derived from hydroxystyrene, and a recurring unit wherein a phenolic hydroxyl group derived from hydroxystyrene is protected by an acid-dissociating, dissolution-suppressing group (e.g. an acetal structure or a tertiary hydrocarbon group) or a recurring unit wherein a carboxyl group derived from (α-alkyl)acrylic acid is protected by an acid-dissociating, dissolution-suppressing group (e.g. an acetal structure or a tertiary hydrocarbon group) (reference is made to, for example, Patent Literatures 1 to 4) and the like. There are also known copolymers having a recurring unit wherein an alicyclic hydrocarbon group is used as an acid-dissociating, dissolution-suppressing group for increasing the dry etching resistance or the difference in dissolution rate in alkaline developing solution before and after light exposure (the difference may be hereinafter referred to as “dissolution contrast”) (reference is made to, for example, Patent Literatures 5 to 6).
In the lithography using an ArF excimer laser of shorter wavelength or the like as the illuminant, there was investigated a copolymer having no recurring unit derived from hydroxystyrene having a high absorption coefficient to a wavelength of 193 nm. As a result, there are known copolymers having, in the recurring unit, a lactone structure (reference is made to, for example, Patent Literatures 7 to 10), or copolymers having, in the recurring unit, a polar group-containing, alicyclic hydrocarbon group (reference is made to, for example, Patent Literature 11), as a polar group for increasing the adhesivity to semiconductor substrate or the like or for controlling the solubility in lithography solvent or alkaline developing solution.
Also, there is known, as a technique for increasing dissolution contrast and enhancing the resolution of lithography, a copolymer having an acid-dissociating structure at the terminal (Patent Literature 12). However, this known technique discloses only a combination with a recurring unit having a structure wherein the hydrogen atom of phenolic hydroxyl group is substituted by the acid-dissociating, dissolution-suppressing group, and is unusable in a lithography using an ArF excimer laser and further has not been sufficient in dissolution contrast.
Incidentally, the method of introducing an acid-dissociating structure at the terminal of copolymer, disclosed in the above known technique is a method of introducing, in anionic living polymerization, an acid-dissociating structure using a polymerization terminator such as haloalkylcarboxylic acid ester having an acid-dissociating structure. Also, as a method of introducing, in radical polymerization, an acid-dissociating structure at the terminal of copolymer, there is considered a method of using a thiol compound having an acid-dissociating structure, as a chain transfer agent. Even in this method, however, when the radical polymerization initiator has no acid-dissociating structure, a terminal structure having no acid-dissociating structure is formed and improvement in lithography properties (e.g. dissolution contrast) is not sufficient.    Patent Literature 1: JP-A-1984-045439    Patent Literature 2: JP-A-1993-113667    Patent Literature 3: JP-A-1998-026828    Patent Literature 4: JP-A-1987-115440    Patent Literature 5: JP-A-1997-073173    Patent Literature 6: JP-A-1998-161313    Patent Literature 7: JP-A-1997-090637    Patent Literature 8: JP-A-1998-207069    Patent Literature 9: JP-A-2000-026446    Patent Literature 10: JP-A-2001-242627    Patent Literature 11: JP-A-1999-109632    Patent Literature 12: JP-A-2003-140350